Large-format battery anodes comprising silicon particles

ABSTRACT

Large-scale anodes containing high weight percentages of silicon suitable for use in lithium-ion energy storage devices and batteries, and methods of manufacturing the same, are described. The anode material described herein can include a film cast on a current collector substrate, with the film including a plurality of active material particles and a conductive polymer membrane coated over the active material particles. In some embodiments, the conductive polymer membrane comprises polyacrylonitrile (PAN). The method of manufacturing the anode material can include preparation of a slurry including the active material particles and the conductive polymer material, casting the slurry on a current collector substrate, and subjecting the composite material to drying and heat treatments.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/407,938, filed Oct. 13, 2016, the entirety of which is herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant numberDE-SC0013852 awarded by the Department of Energy. The government hascertain rights in the invention.

TECHNICAL FIELD

This disclosure relates to energy storage devices such as lithium-ionelectrochemical cells and batteries. More specifically, the disclosurerelates to scalable production of silicon anode electrode sheetssuitable for use in, e.g., lithium-ion energy storage devices andbatteries.

BACKGROUND

Batteries have become ubiquitous in today's society, powering everythingfrom hearing aids to smart phones, forklifts, and even vehicles. Today'sbattery technologies range from heavy, bulky, and inexpensive lead-acidbatteries to lighter, smaller, and pricier lithium-ion batteries (LIBs).Rechargeable LIBs, though, have dominated the portable electronicsmarket for nearly a decade, and recently they have gained significanttraction in the electric vehicle sector and specialty markets includingmilitary applications. Slight improvements in materials processing anddevice manufacturing have allowed for an improvement in energy-densityof approximately 5 to 6% each year, a slow and incremental development.To date, improvements in Li-ion technology have succeeded primarily inmodifying first-generation materials and fitting them into smaller,safer packages. State-of-the-art batteries are still heavy, bulky,expensive, and unsafe, creating barriers to the next-generationapplication power/cost requirements. Reaching the future energy storagegoals will require breakthroughs in next-generation electrode materials.Incorporating higher energy-density active materials is a necessity.

Recently, silicon has been identified as one of the most attractivehigh-energy anode materials for LIBs. Silicon's low working voltage andhigh theoretical specific capacity of 3579 mAh g⁻¹, nearly ten timeshigher than that of state-of-the-art graphite anodes, have encouragedwidespread research efforts aimed at developing a viable Si basedelectrode. Despite the advantages of the Si electrode, a number ofchallenges, mainly associated with the material's severe volumeexpansion, impede its commercialization. While the commercializedgraphite electrode expands roughly 10 to 13% during lithiumintercalation, Si's expansion amounts to nearly 300%, generatingstructural degradation and instability of the all-importantsolid-electrolyte interphase (SEI). Such instabilities ultimatelyshorten the battery life to inadequate levels. Degradation of the activematerial can be mitigated by incorporating materials smaller than 150 nmor through the nanostructural design of electrode architectures capableof reducing expansion. Unfortunately, the electrode architecturespresented in previous works lack sufficiently high coulombicefficiencies largely because the volume change during Si alloying andde-alloying renders the SEI at the Si-electrolyte interface mechanicallyunstable.

Many efforts aimed towards the utilization of silicon in lithium-ionbattery anodes work to combine silicon with conventional activematerials. This provides higher capacity while minimizing the drawbacksof the silicon material (e.g., volume expansion, active materialutilization, etc.). The mixture of nano-silicon (nano-Si) particles instate-of-the-art graphite electrodes has been implemented to commercialpractice to increase the capacity of today's anodes. However, thisprocess is limited to the inclusion of only about 5% (by mass) ofnano-silicon active material. Any amount surpassing the 5% limit willdestroy the electrode's conventional network due to Si's massivevolumetric expansion and contraction during lithiation and delithiation.

Preliminary work performed by Applicants demonstrated the impressivelong-term cycling stability of a nano-Si electrode/room temperatureionic liquid (RTIL or IL) system and its combination with thecommercially available “L333” cathode for a Li-ion cell capable ofdelivering 1.35× the specific energy of today's state-of-the-arttechnology, normalized to electroactive material mass. The nanosilicon-cyclized polyacrylonitrile (nSi-cPAN) electrode, when combinedwith an imide-based RTIL electrolyte, maintains an average half-cellcoulombic efficiency of greater than 99.97% due to the cooperativeeffects of a robust electrode architecture and the formation of a stablesolid-electrolyte interphase (SEI) layer. International Published PatentApplication No. WO 2016/123396, incorporated herein by reference in itsentirety, describes the composition of matter formed during thecombination and utilization of the nSi-cPAN electrode andcomposition-specific RTIL electrolytes in Li-ion batteries.Specifically, the application discloses the composition of the SEIformed between the nSi-cPAN electrode and RTIL electrolyte.

Following the demonstration of the nSi-cPAN system, Applicants developeda “micron-Si” (pSi) anode. The utilization of pSi is made possible byleveraging the mechanical strength of the cPAN coating. By encapsulatingthe pSi particles in a resilient, conductive coating matrix,pulverization of the large Si particles is contained. This mechanism hasbeen dubbed “self-contained fragmentization.” The fragmented siliconparticles remain adhered to the cPAN coating matrix, enabling thelong-term, full utilization of the material with minimal capacitydegradation. This mechanism is validated by the ability of the electrodeto retain its capacity over many cycles, proving that the siliconparticles maintain access to the electronically conductive cPAN matrixeven after pulverization. This is demonstrated in FIG. 1 and describedin International Published Patent Application No. WO 2016/123396, whichdescribes the composition of matter formed by electrochemicallypulverizing large silicon particles within a cPAN matrix.

While the development of the nSi-RTIL system and pSi-cPAN electroderesulted in world-record performance of Li-ion full-cells containingsilicon anodes (high mass loadings of silicon,non-preconditioned/pre-lithiated silicon anodes, long cycle life, highenergy), this performance was only demonstrated at the laboratory scale.The anodes used to demonstrate these inventions, while containinggreater than 70% silicon relative to total anode mass, were thin and notsuitable for commercial application; they were “bench-top”demonstrations made for proof-of-concept and proof-of-viability. Theslurries used to fabricate these anodes contained 12.5 to 25 wt. %solids content (extremely low solids content and not suitable forcommercial manufacturing). Laboratory-standard current collectorsubstrates (greater than 25 to 30 microns thick), low electrode coatingthicknesses (resulting in ˜2 mAh cm⁻²), small electrode areas, and lowcurrents (suitable for coin-type cell demonstration, in the micro-ampererange) allowed for such demonstrations. Translating these technologiesfrom the bench-top to commercial manufacturing lines presents anentirely new set of challenges.

A commercial anode must provide an areal capacity of at least 2 mAh cm⁻²such that it can be paired with a cathode for improved energy densityand cost in large-format Li-ion batteries. This means that thepreviously developed anodes must be scaled (2× in most metrics includingmass loading and thickness to attain attractive energy densities) andprocessed under commercially viable means. Commercial anodes must alsobe developed such that their areal capacity (mAh cm⁻²) remainsconsistent throughout the entire anode sheet in order to properly matchwith the cathode capacity when stacked or wound in pouch or cylindricalcells, respectively. Adhesion between coating and current collectorsubstrate, physical properties of the coating, and even the anodeelectrochemistry change when the anode is scaled to commercial levels.It is well known that it is very difficult to achieve viable, highperforming silicon anodes at aerial capacity loadings greater than 2 to3 mAh cm⁻² and this is especially true for anodes containing high masspercentages of silicon material (greater than 10 wt. %). This is due toadhesion (between electrode and copper current collector substrate) andcohesion (maintenance of the electrode structural integrity within theelectrode itself) issues arising due to the expansion and contraction ofthe silicon active material during lithiation and delithiation,respectively.

SUMMARY

Described herein are various embodiments of a process and composition ofmatter used to facilitate consistent and high-quality commercial scaleSi-cPAN anodes for Li-ion batteries. In some embodiments, the anodescomprise a film cast over a current collector substrate, with the filmcomprising active material particles (e.g., silicon particles) and aconductive polymer membrane coating over the active material particles.In some embodiments, the conductive polymer membrane coating comprises athermoplastic treated to become a cyclized, non-plastic ladder compound.Such anodes can be incorporated into an energy storage device along witha cathode and electrolyte.

Methods of manufacturing the anodes disclosed herein are also described.In some embodiments, the method includes a step of preparing a slurryincluding active material, additive powder, polymer powder and solvent.The slurry is then mixed for a period of time, followed by casting theslurry over a current collector substrate. A drying and heating step arethen carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pair of graphs illustrating cycling data of a pSi-cPANhalf-cell containing a fluorinated electrolyte additive showing therapid CE stabilization achieved through use of Applicant's previouslydisclosed pSi-cPAN/mRTIL system;

FIG. 2 provides high-resolution transmission electron microscopy images(HR-TEM) of nano-spherical silicon particles coated inpolyacrylonitrile.

FIG. 3 is a graph illustrating a full-cell pouch three electrodeexperiment with micron-silicon (anode) and NMC[622] (cathode) workingelectrodes and a lithium counter electrode, with a N/P ratio of 0.9.

FIG. 4 is pair of graphs illustrating pouch full-cell cycling datacomparing impacts of a low N/P ratio to a sufficient N/P ratio.

FIG. 5 provides a graph showing Half-cell cycling performance ofSilicon/PAN anodes highlighting 1 to 3 micrometer silicon particles'(D50 size) performance on a range of coppers with different surfaceroughness, and images of surface morphologies (b)-(e) of various coppermaterials with varying surface roughnesses.

FIG. 6 is a graph showing full-cells (coin) containing silicon/carbonactive material with PAN conductive binder (30 to 35% silicon normalizedto total anode coating mass) and NMC[622] cathodes comparing anodecurrent collector roughness.

FIG. 7 provides a summary of various copper types, their relevantroughness parameters, and a representative surface profilometer spectrumfor the first copper type (“O.M. 10 um (rough)” copper).

FIG. 8 illustrates a Polyacrylonitrile heating process.

FIG. 9 provides SEM and EDS (with mapping) of polymer drivennano-composites comprising silicon and PAN after heat-treatment underargon.

FIG. 10 is a pair of graphs illustrating electrochemical performance ofPAN/Si nano-composite anode heat treated at 300° C. under argon (top)and air (bottom) environment followed by a heat treatment at 600° C.under argon.

FIG. 11 is a graph comparing cyclization in a vacuum oven and underargon flow in a tube furnace, showing high CE and high capacities whencyclized under either environment.

FIG. 12 is a series of graphs showing first cycle voltage profiles offull-cells containing nickel-rich NCM cathodes and Si-cPAN anodes, inwhich the anode component was heat-treated under various procedures.

FIG. 13 is a graph illustrating exemplary half-cell containing a 30 to35% silicon (normalized to total anode mass) anode produced by themethods described herein (6 mAh/cm² loading).

FIG. 14 is a pair of graphs illustrating full-cell single-stack pouchescontaining exemplary anodes as described herein, produced in largebatches suitable for commercial-level performance (commercial massloadings, commercially viable auxiliary components).

DETAILED DESCRIPTION

The following description details various embodiments of methodsassociated with creating commercial scale Si-cPAN electrodes,electrochemical implications associated with some or all of thesemethods, and various embodiments of the resulting compositions ofmatter. The description is divided into sections according to the stepsused to fabricate the electrodes, with each step describing physicalparameters which may be used to obtain improved battery performance.

Conventionally, Si-based electrodes are fabricated with polymer binders(such as polyvinylidene fluoride, polyacrylic acid, styrene-butadienerubber, or carboxymethyl cellulose), conductive additive (usually carbonblack), and Si particles mixed in an organic solvent, such asN-methylpyrrolidone (NMP), to produce a viscous slurry. The slurry isthen bladed onto a copper foil current collector and dried to form ananode electrode. The embodiments described herein relate to fabricationof a polymer driven composite Si anode that differs from theconventional methods in significant ways.

Surprisingly, the methods described herein are compatible withconventional manufacturing infrastructure, allowing for the first true“drop-in” high-loading silicon anode available to the Li-ion market.Other silicon anode production methods are cost and resource intensive,providing significant value to the methods discussed below. As describedin greater detail below, silicon active material is coated in aconductive polymer, such as polyacrylonitrile (PAN), cast on copperfoil, and then treated under heat, and paired with a cathode in aspecific manner so as to enable full-cell performance.

Polyacrylonitrile is discussed herein as an exemplary conductive polymerfor application under the disclosed methods, though other polymers maybe used. Other suitable polymers include, but are not limited to,poly(acrylic acid) (PAA), carboxymethyl cellulose (CMC), and alginate.PAN is a resinous fibrous organic polymer made from mixtures of monomerswith acrylonitrile as the main component. PAN fibers are the chemicalprecursors of high-quality carbon fiber when modified accordingly, andthey are commercially found in a number of high technology and commondaily applications.

A number of active material types can also be utilized under the methodsdescribed herein. Silicon will be discussed as an exemplary anode activematerial for application under this method, with any silicon morphologycapable of integration into the anode slurry and electrode sheet.Silicon morphologies include, but are not limited to, nano-spheres,nano-wires, nano-rods, whiskers, “coral-shaped” silicons,micro-spherical silicon, and various nano-featured large particlesilicon materials. Silicon-graphite, silicon-graphene, silicon-hardcarbon, and other silicon-carbon composite materials are alsonon-exhaustive exemplary anode active materials for application underthe methods described herein. Mixtures of silicon and carbonaceousmaterials such as graphite or hard carbon are also non-exhaustiveexemplary anode active materials.

Lame Batch Slurry Mixing

In order to fabricate the Si-cPAN electrode at scale, the methodsdescribed herein can generally begin by preparing a slurry. The slurryis generally prepared by mixing active materials, polymer, auxiliarymaterials, and additives in a solvent. The resulting slurry preferablyhas specific rheological properties in order to provide highestelectrochemical cycling performance in a Li-ion battery. In someembodiments, the composition of the material added to the solventincludes from about 10 to about 50 wt. % PAN and from about 50 to about90 wt. % active material. In some embodiments, the slurry comprises fromabout 30 to about 60 wt. % solids in from about 70 to about 40 wt. %solvent.

Active material used in preparation of the slurry can includecombinations of materials in different compositions. For example,carbonaceous active materials (graphite, graphene, hardcarbon, etc.) maybe mixed in to form a 10:90 silicon:carbonaceous material weight ratioor 90:10 silicon:carbonaceous material weight ratio. Exemplarycommercial Si:cPAN anodes can include a 30:55:15 Si:Carbonaceousmaterial:PAN weight ratio. Other exemplary weight ratios include 40 to80 wt. % silicon with 5 to 50 wt. % carbonaceous material and 10 to 20wt. % PAN. The carbonaceous material may include mixtures of activematerial and conductive additives including but not limited to carbonblack or carbon nanotubes.

The mixture of active material and conductive binder powder is dispersedin solvent to form a slurry. In some embodiments, the solvent is chosensuch that it is capable of dissolving the conductive binder. Forexample, N,N-dimethylformamide (DMF, 99%) is an exemplary solvent forapplication under the methods described herein when utilizing the PANpolymer. Other suitable solvents include, but are not limited to,dimethyl sulfone (DMSO₂), dimethyl sulfoxide (DMSO),N-methyl-2-pyrrolidone (NMP), N,N-dimethyl acetamide (DMAc), ethylenecarbonate (EC), and propylene carbonate (PC).

Slurry viscosity determines the mixing quality, coating quality, andability to generate large films upon the current collector substrate.Using Brookfield viscosity (spindle 64), exemplary slurry viscositiescan be determined (all measurements taken at room temperature, 23° C.).Slurry viscosity is determined by the solvent/polymer mass ratio andpolymer chain length, as well as by the mass of solvent relative tototal slurry mass. An exemplary slurry made using the materials and themethods described herein exhibits significantly more Newtonian characterthan conventional Li-ion anode slurries. A Newtonian fluid is a fluid inwhich viscous stresses arising from the fluid's flow are linearlyproportional to the strain rate or the range of change in the fluid'sdeformation. This means that as shear force is applied to the slurrydescribed herein, the slurry does not exhibit as much shear thinningrelative to conventional Li-ion anode slurries. This has implications inthe anode mixing and coating process, as the slurry can be mixed at muchhigher RPMs and can be successfully coated at very low thicknesses. Thelow shear thinning nature of the slurry can be studied using aBrookfield viscometer, which is a common instrument in rheologicalcharacterization. In some embodiments, the slurry has a Brookfield ofviscosity ranging between 3000 to 6000 centipoise (cP) between spindlerotations (steel spindle #64) of 12 through 100 RPM at room temperature,varying by less than 1000 cP for a given slurry mixture. The relativelylow variation in viscosity across a range of shear force (described byspindle rotation in RPM) suggests the Newtonian nature of the slurries.Other exemplary slurries range in viscosity from 3500 to 5000 cP at 20to 100 RPM at room temperature.

Slurry mixing parameters are contributors to the performance of theresulting anode. Along with mass percentages, powder mixing, and slurryviscosity, slurry stir time and slurry volume are important factors indetermining the polymer coating quality on the active material. Slurrytime is important to allow for uniform polymer coating over thesuspended active material particles. In some embodiments, slurry mixingtimes of up to 12 hours are sufficient. In some embodiments, lower stirtimes, such as 2 hour stirs, are sufficient with proper equipment.

Slurry mixing can be completed on a variety of equipment. Vacuum andnon-vacuum planetary centrifugal mixers (e.g., “Thinky Mixer” or “ross”mixer), slurry planetary dispersing vacuum mixing machines, doubleplanetary disperser mixers, homogenizers, and simple stirring in abeaker with a stir bar on a stir plate can create adequate mixingconditions.

Slurry volume is important because an adequate amount of material mustbe present to create adequate mixing. Slurry volume also influences theelectrochemical performance of the resulting anodes. If the slurryvolume is too low, a relatively large portion of the material will notundergo stirring/mixing and a uniform coating will not be applied. Forexample, mixtures comprised of 200 mg active material plus polymerpowders and 1.6 g solvent (87.5% total slurry mass solvent) will not mixproperly. There is not enough slurry to create sufficient mixing in thiscase, regardless of mixing method. Slurries utilizing 1.2 g activematerial plus polymer powders in 8.4 g solvent (87.5% total slurry masssolvent) mix well, as do slurries utilizing 1.2 g active material pluspolymer powders in 4-6 g solvent, resulting in uniform polymer coatingover the active material. However, this solids content (12.5 wt. %solids) is not suitable for coating on large scale manufacturingequipment. These slurries will have too low of viscosities and will notbe capable of coating via roll-to-roll methods (e.g., comma bar, slotdie, etc.).

PAN coating quality over the silicon material, resulting from theaforementioned parameters, becomes increasingly important as the anodemass loading increases. Whether adequate coating has been achieved canbe confirmed using microscopy, such as transmission electron microscopy,to identify coating homogeneity and thickness. PAN coatings resulting instrong morphology retention throughout electrochemical cycling should beat least 3 to 5 nanometers thick, found on all surfaces of the activematerial particles, and present throughout the electrode. Examples ofexemplary coatings, formed using the methods described above, are shownin FIG. 2. Electron energy loss spectroscopy (EELS) highlights thesilicon and PAN coating (d). A uniform, 3 to 5 nm coating is present onparticles throughout the electrode matrix.

When preparing the slurry, it may also be suitable to add electrodeadditives that are capable of enhancing performance in full-cell Li-ionbatteries. Exemplary electrode additives include, but are not limitedto, lithium metal powders (such as stabilized lithium metal powder,SLMP) and lithium nitride (Li₃N), as well as other high lithium contentpowders and salts. These dry powders can be added directly to the slurryand mixed along with the other slurry components. Oxalic acid can alsobe added to the slurry to improve dispersion and adhesion properties.

Generally speaking, the polymer dissolves in the solvent. The slurry isthen mixed in order for the polymer material to adequately coat theactive material powder, which is dispersed in the polymer/solventsolution.

Electrode Coating

After mixing a slurry, the slurry is cast on a current collectorsubstrate via, e.g., roll-to-roll coating methods. Roll-to-roll coatingmachines can be used to create hundreds of meters of electrode in singleruns. The roll-to-roll coating process is determined by the physicalproperties of the slurry (e.g., shear, viscosity, etc.). The currentcollector foil is drawn through the coater at a speed of 0.2 to 50meters/minute, and the coated foil passes through a drier set to atemperature of from 30 to 70° C. for water based slurries and from 110to 160° C. for solvent-based slurries. In the case of the technologydescribed by this application, the drier temperature should be set to 30to 70° C. despite being a solvent-based system.

An important factor to electrode coating is the resulting areal massloading of silicon per square centimeter, the capacity this provides,and how these numbers match to the cathode utilized in the full cell. Inorder to pair with high energy cathode materials, including the“nickel-rich NMC” (nickel manganese cobalt oxide cathodes), the arealcapacity of the anode should be from 1.3 to 2.0× the areal capacity ofthe cathode. This factor is known in the industry as the “N/P ratio”(negative electrode capacity/positive electrode capacity). Inconventional Li-ion cells containing graphite anodes, the N/P ratio istypically from 1.1 to 1.2, and is set to avoid lithium plating on theanode during cycling. The N/P ratio devised for the system describedherein is designed to cover efficiency losses during initial cycling andto “pin” the anode half-cell voltage between 0.01 and 1.5 V vs. Li/Li⁺.If the N/P ratio is too low, the anode half-cell voltage falls below0.01 V (due to full- and over-lithiation of silicon) and the anode willbe destroyed. This is illustrated in FIG. 3. FIG. 4 shows a cell with apoor N/P ratio compared to a cell with a strong N/P ratio. The higherrelative N/P ratio also prevents wrinkling and deformation of the anodefilms due to the expansion and contraction of the silicon materialpresent in the films. A desirable N/P ratio depends on the silicon massloading relative to the total anode film mass. If the anode filmcomprises 20 to 50 wt. % silicon, the N/P ratio should be 1.2 to 1.6. Ifthe anode film comprises greater than 50 wt. % silicon, the N/P ratioshould be greater than 1.6. In some embodiments, the weight percent ofsilicon in the anode can be added to “1” to obtain the minimum N/P ratiofor the system. In other words, if the anode comprises 40 wt. % silicon,the N/P ratio for the resulting full-cell system should be greater than1.4.

Current Collector Substrate

The current collector substrate, typically a metal foil, is used to moveelectrons from outside the cell to the electrodes, and vice versa. Theelectrode slurry is cast onto the foil in a coating of uniformthickness. In order for the Li-ion battery to function properly, theelectrode coating should be adhered adequately to the current collectorfoil and should maintain this adherence throughout cycling. In alloyingelectrodes, such as silicon, this is especially difficult when usinglarge format electrodes (which tend to be very thick) given theexpansion properties of the active material. The conductive binder isresponsible for adhering the anode film to the current collectorsubstrate; in large format electrodes it is necessary to have sufficientbinder present to allow for adherence. In the silicon plus PAN systemdescribed herein, 10 to 25% PAN, relative to total anode coating mass,is the minimum polymer content allowable. This is unique to scaled,large format silicon anodes with thickness greater than 5 micrometers.

Significantly and surprisingly, the physical properties of the currentcollector substrate are of high importance to the performance of theresulting anode sheets. Along with anode composition, the physicalproperties of the current collector substrate allow for cell longevity.Copper foil is an exemplary current collector substrate and is mostcommonly used in conventional Li-ion cells. Copper foil properties arediscussed herein.

An important physical property in the copper relative to anode filmadhesion is surface roughness. One measure of roughness is ten-pointheight or maximum height (R_(z)). This is a root mean square value.R_(z) is defined as the average of the peak-to-valley numbers in a givenscan area, with at least five consecutive points measured (five highestpeaks+five highest values=10 points). For some embodiments of the anodesdescribed herein to maintain adherence throughout cycling, the copperR_(z) should be at least 1.5 micrometers. Other embodiments with largeractive materials may require higher surface roughness up to 6 to 7micrometers. Other embodiments with nanoscale active materials requireR_(z) values greater than 0.5 micrometers. Copper foils used inconventional/previously commercialized Li-ion anodes commonly have R_(z)values below at or below 0.5 microns.

Another measure of surface roughness is the arithmetical mean height(Sa) which expresses the magnitude of the difference in height of eachpoint compared to the arithmetical mean of the surface. Yet anothermeasure of surface roughness is the developed interfacial area ratio(Sdr) which is the percentage of the area's surface contributed by thetexture as compared to the planar definition area (i.e., a completelylevel surface has Sdr=0). Each of these parameters, along with theirrelative magnitudes, are important to performance of the systemdescribed herein.

As mentioned previously, required copper surface roughness for viableperformance is highly dependent upon active material size. Current datasuggests that if electrode film expansion is below 50% in the z-axis(normal to the electrode substrate) and the active material particlesize is above 500 nanometers, copper with surface roughness R_(z)greater than 0.5 micrometers provides the best performance. If electrodefilm expansion is above 50% in the z-axis (normal to the electrodesubstrate) and the active material particle size is above 500nanometers, copper with surface roughness R_(z) greater than 2micrometers provides the best performance.

Unfortunately, the higher surface roughness is associated with anecessary higher thickness, which is detrimental to battery energydensity as the thicker current collector (an auxiliary material thatdoes not contribute to cell capacity) takes up space and does notprovide energy. Improved adherence on rougher copper surfaces isexplained by increased surface area available for adherence between theconductive polymer and the copper. With reference to FIG. 5, variouscopper surfaces were imaged under an optical microscope for comparison.

Performances of half-cells containing a silicon plus PAN anode are alsoshown in FIG. 5, demonstrating the clear need for appropriate coppersurface roughness to maintain adherence to the anode film during manycharge-discharge cycles, based on the active material morphology andanode film structure. In half-cells, the first cycle coulombicefficiency (CE) is higher and CEs stabilize much more quickly in cellscontaining rougher copper foils; this is attributed to maintainedelectronic contact in the cell and faster electron transport. The CEbehavior and associated electron transport/adhesion properties manifestin higher performing full-cells with active material particles of sizegreater than 500 nanometers, as shown in FIG. 6. In full-cellscontaining anode current collector films with R_(z) at or under 1microns, cell performance gradually declines and eventually crasheswithin 50 cycles.

A summary of various copper types, their relevant roughness parameters,and a representative surface profilometer spectrum for the first coppertype (“O.M. 10 um (rough)” copper) are provided in FIG. 7. The coppertypes notated with shaded font (“O.M. 10 um (rough)” and “VL10|23 um”coppers) provide the best performance across a range of silicon materialtypes and anode film microstructures. The materials shown in FIG. 7generally perform the best with the scaled electrode system describedherein. As such, a new parameter, Sa/Sdr, is used to describe the bestperforming copper foils based on roughness parameters. Sa/Sdr describesthe ratio in magnitude of the mean height of peaks and valleys in thefoil surface to the percentage of surface caused by roughness. In otherwords, a high Sa/Sdr means that the copper surface has very high peaksrelative to the total roughness and have a low frequency, where theSa/Sdr near 1 suggests that the copper surface peak/valley heights aremore equally distributed. The Sa/Sdr nearer to 1 is found to befavorable, with Sa/Sdr under 3 found to be sufficient for highperformance with a range of silicon material types and anode filmcompositions and microstructures.

Electrode Calendaring

In some embodiments, the electrode is dried after coating on theroll-to-roll coating equipment with the drier temperature should be setto 30 to 70° C. with air flow. After slurry casting on current collectorfoil and solvent drying/evaporation, conventional graphite anodes arecalendared to about 70% of their original film thickness. Thiscalendaring results in porosities of about 40 to 50%. Such a processprovides higher degrees of particle contact while still allowing foradequate electrolyte penetration. Beyond 50% porosity, conventionalanodes lack sufficient mechanical strength to withstand batteryproduction and operation. The system described herein is different. Theelectrode requires higher porosity in order to accommodate the volumeexpansion of the silicon material, and higher inter-film surface area isadvantageous for formation of a robust SEI layer and faster Li⁺ iontransport. The anodes described herein, including the silicon plus PANanode, are calendared to a porosity of 40 to 70%. Exemplary porositiesof the Si-cPAN composite will be 50 to 60%. This compares to traditionalgraphite anodes, which are about 30 to 40% porous. Traditionalelectrodes containing silicon, which today range in up to 15 wt. % Si,(PAA, CMC, SBR, etc. binders) have porosities ranging from 40 to 50%.Active materials showing higher degrees of expansion require electrodeswith higher porosities.

Electrode Heat Treatment

An important aspect of the anode described herein lies in the ability ofa conductive polymer binder to act as both a binder material and anelectronically conductive matrix capable of providing efficient chargetransfer throughout the composite. Creating a high performing anode filmof this type at scale is indeed difficult and requires the understandingof many intricacies, as described above, but another layer of complexityis added by the need to treat many conductive polymers in order toattain electronic conductivity. The exemplary conductive polymerdiscussed herein (e.g., PAN) can be heated in a reducing atmosphere orunder vacuum in order to exhibit electronic conductivity. At the sametime, the polymer can be treated such that the polymer matrix does notbecome too brittle for the mechanical effects of battery cycling (causedby heat treatment at very high temperatures or under inadequateatmosphere). Moreover, the treatment should be conducted such that theauxiliary components of the electrode (i.e., copper) are not affectedand remain in condition for battery operation.

Many conductive polymers are capable of chemical transformationresulting in electronic conductivity, however most need the addition ofcross-linkers to catalyze these chemical reactions. PAN and copolymersof PAN are unique in that they are self-catalytic through heat treatmentprocesses. PAN is a unique linear semi-crystalline organic polymer withmolecular formula (C₃H₃N)_(n). PAN's molecular structure is composed ofcarbon chains with coordinated nitrile groups. The chemistry of PAN isof particular interest because of PAN's unique self-catalyticcyclization reaction and cross-linking through thermal stabilization.PAN's chains degrade before they reach the melt state and thedegradation process, commonly referred to as “cyclization,” turns linearPAN chains into thermally stable conjugated ladder-like structures thatdo not flow or dissolve. This is illustrated in FIG. 8. This thermalstabilization allows the fibers to withstand carbonization andgraphitization temperatures (about 1000 to 3000° C.) and yield highperformance carbon fibers without excessive weight lost or chainscission.

Thermal stabilization of PAN refers to the low temperature (generally200 to 300° C.) conversion of the polymer fibers to a high temperatureresilient fiber. The conversion is necessary for the fibers to survivecarbonization (800 to 1300° C.) and graphitization (1300 to 3000° C.)with the highest possible carbon yield and superior properties. Themajor chemical reactions involved in this process are known ascyclization, dehydrogenation, oxidation, and cross-linking which resultin the formation of a thermally stable conjugated ladder structure.

Cyclization is an important reaction during the stabilization of PAN anda main focus for the treatment of the anode described herein.Cyclization takes place when the nitrile bond (C≡N) reacts to developcrosslinking between molecules of PAN, creating a double bond (C═N) anda stable conjugated ladder polymer of fused pyridine rings. The thermalstability of the stabilized fiber is attributed to the formation of theladder structure by cyclization of the nitrile groups, enabling theoperation of stabilized PAN at high temperatures with minimumvolatilization of carbonaceous material. Cyclization is the reason whystabilized fibers change color from white to yellow to brown to black.Cyclization is exothermic and has the potential to damage the fibers ifdone too quickly. The fibers can shrink excessively, lose significantmass, and even melt and fuse together. Conversely, if the stabilizationprocedure is too conservative (both with time and heat) the fibers willonly partially stabilize. Unlike dehydrogenation, cyclization does notneed the presence of oxygen to take place, so it can occur in an inertatmosphere. The reaction atmosphere is important in the method describedherein since the anode copper foil current collectors are involved inthe heat treatments of PAN and any exposure to oxygen at temperaturesabove 100° C. will oxidize the foils creating deficiency issues(electronic resistances and electrochemical side reactions).

In embodiments described herein, PAN is treated only to its stabilized(specifically ‘cyclization’) stage and then the resulting pyridine-basedconjugated polymer is applied as an electrode binder/coating with robustmechanical properties as well as intrinsic electronic properties. Thereason from deviating from the conventional stabilization procedure forhigh carbon yield and high performance carbon (oxidation,dehydrogenation, and cyclization to perform carbonization andgraphitization) is to avoid the formation of a highly oriented (aligningof basal planes), stiff, and brittle coating around an active materialprone to high degrees of expansion and contraction.

The stabilization of PAN from linear molecules to ladder polymercompounds by cyclization can be conducted by heating in an inertenvironment from 100 to 500° C., with an exemplary temperature for PAN'scyclization heat treatment at 300° C., at a rate of 5° C./min with 2 to12 hr hold time at the peak temperature. Running a range of temperaturesallowed for determining the best peak for the electrochemicalperformance of a PAN/Silicon configuration. FIG. 9 shows scanningelectron microscopy (SEM) images and energy dispersive spectroscopy(EDS) analysis on some of these polymer driven nano-composite samples.PAN/silicon samples were tested using the above-described methods andothers were subsequently treated for the carbonization of PAN and heldfor 1 hr at peak temperatures of 500° C., 600° C., 700° C., 800° C.,900° C. and 1000° C. under an the same argon atmosphere. Again, theheating rate of 5° C./min was kept for this second stage of heat aswell. FIG. 10 and FIG. 11 illustrate electrochemical data of variousheat treatment tests.

In addition to the treatment parameters of heating time and temperature,the equipment utilized and resulting atmospheric conditions areimportant. The anode should be treated under vacuum or inert gas flow,as previously discussed. Argon and nitrogen are exemplary inert gasatmospheres, with pressures of 20 to 80 PSI providing the mostconsistent heating conditions. Heat treatment under vacuum also allowsfor adequate cyclization conditions and high performing anodes. Theseatmospheres can be provided in a range of equipment types, includingtube furnaces, gloveboxes, vacuum ovens, or other atmosphere controlledovens. Gas flow during heat treatment allows for improved heating andelectrochemical performance as the by-products of the polymer chemicalreaction (including hydrogen off-gassing during PAN cyclization) areflushed from the system and therefore cannot react with the electrode orcurrent collector substrate. Gas flow through the tube furnace should beset at a rate of 100 to 1,000 liters per hour for best performance. Anentire roll of anode, or multiple rolls of anode, can be treated in asingle oven. An atmosphere controlled furnace can also be added to theindustry standard roll-to-roll coating system utilized for electrodefabrication. Typically the electrode travels through a drying ovenimmediately after coating; the electrode could also be passed through anatmosphere controlled furnace following initial drying with conditionsset to induce polymer chemical transformation (i.e., PAN cyclization)before being rolled onto a spool. In lieu of this equipment, tubefurnaces provide a commercially viable means of treating large rolls ofanode, including the Silicon/polyacrylonitrile composite.

Of particular importance, especially as it relates to commercial scalemanufacturing, is the finding that heating time and heat ramp rateshould be tuned according to the anode microstructure and size.Specifically, anode thickness, PAN weight percentage, and the amount ofanode material undergoing treatment affect these processing parameters.As represented in FIG. 12, heating times of just 2 hours can besufficient for proper electrochemical performance.

Electrochemical Performance

FIG. 13 presents exemplary data for a micron-Silicon:polyacrylonitrile(Si:PAN) anode half-cell fabricated through the methods describedherein. The mass loadings, processing, and resulting composition ofmatter are sufficient for commercial use. FIG. 14 presents exemplarydata for a full-cell comprising the exemplary anode described herein anda “nickel-rich” high-energy cathode (“NMC[622]”).

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thescope of the invention. Accordingly, the invention is not limited exceptas by the appended claims.

What is claimed is:
 1. An energy storage device comprising: a cathode;an electrolyte; and an anode comprising a film having a thickness of 10to 80 micrometers cast over a current collector substrate with a surfaceroughness R_(z) of greater than 1.5 microns, the film comprising: A) aplurality of active material particles, wherein the active materialparticles include at least one of silicon, hard-carbon, graphite,graphene, germanium, titanium oxide, tin, magnesium, antimony, and lead;and B) a conductive polymer membrane coating over the active materialparticles, the conductive polymer membrane coating comprising athermoplastic polymer treated to become a cyclized, non-plastic laddercompound.
 2. The energy storage device of claim 1, wherein the anodecomprises 30-60 wt. % silicon particles and the areal gravimetriccapacity of the anode is 1.3 to 1.6 times that of the cathode's arealgravimetric capacity.
 3. The energy storage device of claim 1, whereinthe anode comprises greater than or equal to 60 wt. % silicon particlesand the areal gravimetric capacity of the anode is 1.6 to 2.0 times thatof the cathode's areal gravimetric capacity.
 4. The energy storagedevice of claim 1, wherein the thermoplastic polymer treated to become acyclized, non-plastic ladder compound comprises polyacrylonitrile. 5.The energy storage device of claim 1, wherein the electrolyte comprisesan imide-based room temperature ionic liquid.
 6. The energy storagedevice of claim 1, wherein the porosity of the anode film is between50-70%.
 7. The energy storage device of claim 1, wherein the magnitudeof the arithmetical mean height Sa is less than three times thedeveloped interfacial ratio Sdr.
 8. A method of making an anodecomprising a film thickness of 10 to 80 micrometers cast over a currentcollector substrate, the film comprising a plurality of active materialsand a thermoplastic polymer treated to become a cyclized, non-plasticladder compound, the method comprising: A) preparing a slurry having aBrookfield viscosity of 2000-6000 cP at 20 to 100 RPM using a #64spindle, at room temperature, by placing a mixture of active material,additive powder and polymer powder in a solvent capable of dissolvingthe polymer powder; B) mixing the slurry for a time of 1 to 4 hours; C)casting the slurry over a current collector substrate; D) drying thecasted film; and E) applying heat to the casted film at temperatures of200 to 400° C. for a time of 1 to 12 hours.
 9. The method of claim 8wherein the active material includes at least one of silicon,hard-carbon, graphite, germanium, titanium oxide, tin, magnesium,antimony, and lead.
 10. The method of claim 8 wherein the thermoplasticpolymer treated to become a cyclized, non-plastic ladder compoundcomprises polyacrylonitrile.
 11. The method of claim 8 wherein theadditive powder comprises lithium metal powder.
 12. The method of claim8 wherein the additive powder comprises lithium nitride.
 13. The methodof claim 8 wherein the additive powder comprises oxalic acid.
 14. Themethod of claim 8 wherein the application of 200 to 400° C. heat iscompleted under vacuum or inert gas flow.
 15. An energy storage devicecomprising the anode manufactured according to the method of claim 8, acathode, and an electrolyte, wherein the areal gravimetric capacity ofthe anode is 1.3 to 2.0 times that of the cathode's areal gravimetriccapacity.